It may not come as a surprise that women who are underweight have trouble conceiving, since they sometimes struggle with irregular periods. What's more surprising is that even IVF, which doctors thought would compensate for these problems, is still less effective in underweight women than in those at a healthy weight, or those who are obese.

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The Telegraphreports on a study of nearly 2,500 IVF cycles at a Chicago clinic. Researchers found that 50% of cycles carried out on women who were at a healthy weight or slightly overweight (as defined by BMI) resulted in the birth of a child. For obese women, the number was just a bit lower, at 45%. But for women who were underweight, the portion of cycles that led to a live birth dropped to 34%. Lead study author Richard Sherbahn said he was surprised by the results:

There has been the suggestion from other observations that being too thin is not good for fertility. Women that are too thin sometimes don't always get periods or have irregular periods and it alters their hormonal situation. But that shouldn't impact on IVF because we're giving them hormones, we're making them ovulate and we're getting eggs from them. I was kind of surprised, I didn't expect these findings.

The research suggests that just producing eggs might not be enough to conceive. Gynecologist Charles Kingsland tells the Telegraph "that thin women may not produce sufficient hormones to produce a womb lining that can sustain a pregnancy." He adds, "It's a general health issue — if a woman has a BMI of 16 and comes along with a fertility problem, IVF is not going to cure her fertility problem." It makes a certain amount of sense that if a woman's body is in starvation mode, it might have trouble supporting a pregnancy even with the help of IVF. And though obese women's pregnancies have gotten a lot of attention in recent years, the new study suggests that underweight women may actually face more problems, at least where fertility is concerned.